Comparison of the sulfur hexafluoride tracer and respiration chamber techniques for estimating methane emissions and correction for rectum methane output from dairy cows

Abstract

The objectives of the present study were to compare the sulfur hexafluoride (SF6) and respiration chamber techniques for measuring methane (CH4) emissions from dairy cows and to determine the proportion of CH4 that is released through the rectum. Data used were derived from 20 early lactation dairy cows in a 2 × 2 factorial design study for 4 periods with 6 wk/period. The 4 treatment diets consisted of grass silage and 2 levels of concentrate (30 and 60% dry matter basis), with or without yeast supplement. At the end of each period, CH4 emissions were measured simultaneously using the SF6 and respiration chamber techniques when cows were housed in chambers. The SF6 technique was also used when cows were housed in digestibility units (barn location) before and after respiratory chamber measurements (chamber location). The simultaneous measurements in chamber location revealed that CH4 emission estimates by the SF6 technique were similar to those by the respiration chamber technique in the first 3 periods, although the SF6 estimates were significantly higher in period 4. The regression of all data from the 4 periods demonstrated a linear relationship between the SF6 and respiration chamber measurements for total CH4 emissions (g/d, R2=0.69) and for CH4 emissions per unit of milk yield (g/kg, R2=0.88), and a quadratic relationship for CH4 emissions per unit of dry matter intake (g/kg, R2=0.64). The CH4 emissions from the rectum were calculated as the difference between CH4 estimates from the SF6 technique when cows were housed in respiratory chambers and barn locations, which was 3% of the total CH4 emissions from the mouth, nostrils, and rectum. The SF6 estimates in the chamber location accounted for all sources of emissions, whereas those in the barn location, like that in grazing conditions, did not include CH4 emission from the rectum. Therefore, the SF6 measurements for grazing cattle should be adjusted for CH4 emissions from the rectum (3% of total). We conclude that the SF6 technique is reasonably accurate for estimating CH4 emissions.